The present invention is directed to a prism coupler for coupling radiation into and out of an optical waveguide by utilizing a coupler composed of a prism having a base surface with a coupling surface extending from the base surface at a prism angle. The prism is positioned with the base surface in contact with the waveguide and the prism angle and index of refraction of the prism will determine the coupling angle to the normal of the coupling surface for coupling radiation into and out of the waveguide.
In the coupling of light into and out of an optical waveguide by use of a prism, the coupling angle .alpha. as a rule is temperature-dependent.